Some metal-clad laminates are produced by bonding an insulating substrate and a metal layer to each other through an adhesive or an adhesive film. For example, there has been proposed a meta-clad laminate having a three-layer structure which is formed by bonding an insulating substrate constituted from an aromatic polyimide resin film and a metal layer to each other through an adhesive film (refer to Patent Document 1).
Hitherto, the adhesives or adhesive films made of epoxy resins or acrylic resins have been mainly used. However, since these resins are poor in heat resistance, products obtained after adhesion tend to show an insufficient heat resistance, to limit the conditions for subsequent processing and use.
Under these circumstances, there is a demand for adhesives and adhesive films having an excellent heat resistance. For example, there are disclosed a method of forming a heat-pressure-sensitive adhesive layer by applying a solution of a polyimide resin or a polyamic acid onto an insulating substrate, removing the solvent and then optionally conducting an imidation treatment; a method of forming a heat-pressure-sensitive film by applying a solution of a polyimide resin or a polyamic acid on a substrate such as a glass plate, removing the solvent and then optionally conducting an imidation treatment; and a method of heat-press bonding the adhesive layer or adhesive film to an adherend such as a metal layer (refer to Patent Documents 2 and 3). The methods for forming the adhesive layer or film are roughly classified into methods using a solution of a polyimide resin and methods using a solution of a polyamic acid.
In the methods using the polyamic acid solution, to convert the polyamic acid solution which is applied on an insulating substrate into an adhesive layer or adhesive film, an imidation step at a high temperature exceeding 300° C. must be conducted. If the imidation step is omitted, the resultant metal-clad laminates have an extremely low heat resistance. In the methods using the polyimide resin solution, an adhesive layer or adhesive film is produced only by evaporating the solvent at a temperature as low as up to about 200° C. Therefore, the methods using the polyimide resin solution are more advantageous in the production of high-heat resistant metal-clad laminates. Many of the conventional adhesive layers made of wholly aromatic polyimide resins have been produced using the polyamic acid solution. To prepare a solution, the polyimide resin must be solvent-soluble.
In the conventional metal-clad laminates, when the amount of residual volatile components in the adhesive layer interposed between the insulating substrate and the metal layer is large, the adhesive layer tends to suffer from whitening, swelling, foaming, etc. during a soldering step where the temperature reaches 250° C. or higher, to considerably impair the adhesion between the insulating substrate and the metal layer (refer to Patent Document 4). The residual volatile components in the adhesive layer include water and solvent which are not removed in the imidation step for forming the adhesive layer or adhesive film and the solvent removal step, water absorbed from surrounding atmosphere during the production, and water absorbed when immersed in an aqueous solution during an etching step. Among these volatile components, water is especially problematic. To solve the problem, it is required to lower the water absorption of polyimide which governs the water content.
There is also disclosed a thermoplastic polyimide resin containing a 1,2,4,5-cyclohexanetetracarboxylic acid skeleton in its main molecular chain which is produced from 1,2,4,5-cyclohexanetetracarboxylic dianhydride or its reactive derivative (refer to Patent Document 6). In Example 1 of Patent Document 6, there is disclosed a transparent yellow polyimide resin film having a glass transition temperature of 304° C., which is produced by reacting 1,2,4,5-cyclohexanetetracarboxylic dianhydride or its reactive derivative with diaminodiphenylmethane to obtain an amic acid, imidating the amic acid under heating after coating, and then heat-pressing the imidated product into film. In addition, in Patent Document 5, it is described that a less-colored, transparent polyimide resin film having a glass transition temperature of 300° C. or higher is obtained from a polyimide resin solution in diaminodiphenyl ether.
The polyimide resin having a 1,2,4,5-cyclohexanetetracarboxylic acid skeleton is advantageous in view of forming and processing, because its molecular weight is relatively easily increased, it is easily formed into a flexible film, and its solubility in solvents is sufficiently high. Further, such a polyimide resin is extremely advantageous because a flexible adhesive layer having a sufficient thickness and a good durability is easily obtained by a coating method.
However, the polyimide resin film described in Patent Document 6 is discolored because the film is formed through a high-temperature imidation step as in the conventional methods. In addition, the polyimide resin films described in Patent Documents 5 and 6 have a high water absorption, and therefore, show a poor dimensional stability upon moisture absorption.    Patent Document 1: JP 55-91895A    Patent Document 2: JP 5-32950A    Patent Document 3: JP 5-59344A    Patent Document 4: JP 2001-329246A    Patent Document 5: JP 2003-168800A    Patent Document 6: U.S. Pat. No. 3,639,343